http.StatusTooManyRequests breaks Go 1.5 compatibility
This constant, that we use as of 0.8.3, only exists in Go 1.6 and newer.
there are five new error codes: StatusPreconditionRequired (428), StatusTooManyRequests (429), StatusRequestHeaderFieldsTooLarge (431), and StatusNetworkAuthenticationRequired (511) from RFC 6585, as well as the recently-approved StatusUnavailableForLegalReasons (451).
We need to replace that constant and test if everything else still compiles on 1.5.
cc @ayufan